Cyclic deformation behaviour of austenitic steels at ambient and elevated temperatures

The aim of the present investigation is to characterise cyclic deformation behaviour and plasticity-induced martensite formation of metastable austenitic stainless steels at ambient and elevated temperatures, taking into account the influence of the alloying elements titanium and niobium. Titanium and niobium are ferrite-stabilising elements which influence the ferrite crystallisation. Furthermore, They form carbides and/or carbonitrides and thus limit the austenite-stabilising effect of carbon and nitrogen. Several specimen batches of titanium and niobium alloyed austenite and of a pure Cr-Ni-steel for comparison were tested under stress and total strain control at a frequency of 5 Hz and triangular load-time waveforms. Stress-strain-hysteresis and temperature measurements were used at ambient temperature to characterise cyclic deformation behaviour. Plasticity-induced martensite content was detected with non-destructive magnetic measuring techniques. The experiments yield characteristic cyclic deformation curves and corresponding magnetic signals according to the actual fatigue state and the amount of martensite. Fatigue behaviour of X6CrNiTil810 (AISI 321), X10CrNiCb189 (AISI 348) and X5CrNi1810 (AISI 304) is characterised by cyclic hardening and softening effects which are strongly influenced by specific loading conditions. Martensite formation varies with the composition, loading conditions, temperature and number of cycles.     

Fretting fatigue behaviour of shot-peened Ti-6Al-4V at room and elevated temperatures

Fretting fatigue behaviour of shot‐peened titanium alloy, Ti‐6Al‐4V was investigated at room and elevated temperatures. Constant amplitude fretting fatigue tests were conducted over a wide range of maximum stresses, σ_max= 333 to 666 MPa with a stress ratio of R= 0.1 . Two infrared heaters, placed at the front and back of specimen, were used to heat and maintain temperature of the gage section of specimen at 260 °C. Residual stress measurements by X‐ray diffraction method before and after fretting test showed that residual compressive stress was relaxed during fretting fatigue. Elevated temperature induced more residual stress relaxation, which, in turn, decreased fretting fatigue life significantly at 260 °C. Finite element analysis (FEA) showed that the longitudinal tensile stress, σ_xx varied with the depth inside the specimen from contact surface during fretting fatigue and the largest σ_xx could exist away from the contact surface in a certain situation. A critical plane based fatigue crack initiation model, modified shear stress range parameter (MSSR), was computed from FEA results to characterize fretting fatigue crack initiation behaviour. It showed that stress relaxation during test affected fretting fatigue life and location of crack initiation significantly. MSSR parameter also predicted crack initiation location, which matched with experimental observations and the number of cycles for crack initiation, which showed the appropriate trend with the experimental observations at both temperatures.     

Cyclic fracture behaviour of 20MnMoNi55 steel at room and elevated temperatures

Cyclic fracture behaviour of 20MnMoNi55 steel has been evaluated at room temperature and elevated temperature of 300 °C. Fracture resistance at elevated temperature was found to be lower than that at room temperature. The deterioration in fracture resistance was due to dynamic strain ageing. Cyclic loading amplitude and frequency were also found to have an influence on fracture resistance. The features of the stretch zone during cyclic fracture were different from those at monotonic fracture. Stretch zone dimensions were found to be insensitive to cyclic load amplitudes whereas marginally affected by frequency of load cycles under dynamic strain ageing conditions.     

Low cycle deformation behaviour of Zircaloy-4 at elevated temperatures

The LC deformation behaviour of Zry-4 at 400°C and 600°C was examined by means of tension/compression experiments conducted in load and in strain control respectively. The main results were compared to those obtained at comparable conditions on the stainless steel type AISI 304. For both the materials the influence of the stress ratio R = σ_min/σ_max (where within one test σ_max > 0 was kept constant) upon the lifetime T_f at low and high homologeous temperature T_h was examined. Whereas at the lower T_h for R < 0 the lifetime decreased with decreasing R, the opposite was true at the higher T_h. The explanation of the influence of R upon t_f suggests that at high temperatures the fatigue damage rate Å_f drops below the rate for creep damage Å_c Two cases are considered. If the above damage mechanisms are sequentially independent the resulting damage rate Å ≈? Å_c and hence Å_c is the failure (rate) determining mechanism. In the case that the mechanisms are sequentially dependent then Å ≈? Å_f. TEM investigations conducted on Zry-4 cycled at 600° C have shown that the typical dislocation pattern revealed is a band structure consisting of dense dislocation walls separating denuded zones. The habit and the crystallographic characteristics of the band structure resemble the structure associated with PSBs observed in fee metals. The comparison of the values of the saturation stress τ_s and the wall spacing d for different fee and hep metals shows that there is a proportionality between τ_s and 1/d which is independent of stress and temperature.     

Hypervelocity impact tests on coated thermoplastic films at cryogenic and elevated temperatures

The hypervelocity impact facility at Space Research Institute (SRI), Auburn University has recently completed a series of tests on coated thermoplastic films at cryogenic (40 K) and elevated temperatures (420 K). With a 1-in gap, two films were mounted in a frame that applied biaxial tension to each film. The materials were impacted with 40–100 μm soda lime spheres utilizing a plasma drag gun to accelerate the particles to velocities between 5 and 12 km/s. The facility diagnostics allow for the determination each particle's, size, velocity and impact location along with micrographs of the nature of the impact damage. This summary of the test includes a general overview of the nature of damage on the films along with representative impact micrographs of the impact sites.     

Interaction of Si3N4 with titanium at elevated temperatures

In relation to the joining of silicon nitride ceramics to metal, the reaction products and the reaction mechanism between Si3N4 and titanium have been investigated under a nitrogen or an argon atmosphere at temperatures of 823–1573 K. Using Si3N4/titanium powder mixtures, reaction rates were determined by thermogravimetric (TG) analysis, and reaction products were examined by X-ray diffraction. At higher temperatures and on prolonged heating, reaction products were changed in the following orders: TiN2, TiN2+TiN, TiN+TiSi2+Si, TiN+Si and TiN (nitrogen atmosphere) and TiN2+Ti5Si5, TiN2+TiN+Ti5Si3 and TiN+TiSi2+Si (argon). By relating these results to TG measurements, a full understanding of the reaction mechanism between Si3N4 and titanium was acquired. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of carbon concentration on tensile behaviour of pearlitic steels

Abstract

A quantitative relationship between flow stress and microstructure is studied for pearlitic steels incorporating 0.39 - 0.77 wt-% carbon. The distribution of true lamellar spacing (S ₀) is determined. It is found that S ₀ depends on carbon concentration and pearlite transformation temperature accompanying a considerable distribution. The 0.2% proof stress is described as a function of the averaged S ₀ but the influence of the accuracy in S ₀ measurement precludes satisfactory prediction of the 0.2% proof stress. High work hardening corresponds to the generation of phase stress caused by misfit plastic strain between ferrite and cementite. The stress partitioning behaviour between ferrite and cementite is verified by in situ neutron diffraction during tensile deformation.     

Fracture and strength properties of selected austenitic stainless steels at cryogenic temperatures

Austenitic stainless steels have an excellent combination of mechanical and physical properties for load-bearing structures of large superconducting magnets for plasma containment in magnetic fusion experiments. To assess their relative suitability fracture toughness, fatigue crack growth, and tensile properties data for five austenitic steels at 295, 76, and 4 K have been obtained. The steels were AISI 304, 316, 304LN, and 316LN, and an Fe-21cr-12Ni-5Mn alloy with a higher nitrogen content than the other four grades. The two principal findings were the systematic variation of yield strength with nitrogen content and a systematic inverse correlation between fracture toughness and yield strength. Data from previous studies are reviewed which confirm the trends of the present data.     

Austenitic stainless steels at cryogenic temperatures: The compositional dependence of the Ms

The problem of predicting the likelihood of martensitic transformations in austenitic stainless steels at low temperatures remains an important one, in which there is still much uncertainty. Spontaneous transformations in the absence of plastic strains have generally been analysed in terms of the Eichelman and Hull equation or its variants. This equation is useful in that its predictions are qualitatively correct when applied to the AISI 300 stainless steels but are not really accurate enough for deciding where the dividing line between transforming and non-transforming steels lies in any one grade of steel. A re-examination of the existing M_S data shows that the elemental coefficients in linear equations are approximate and that a quadratic equation is more likely. Our analysis shows that the exact range of validity of the Eichelman and Hull equation is more restricted than thought hitherto and that the elemental coefficients show systematic deviations as the composition changes towards the practically important steels in the AISI 300 grades of steel.